1. Field of the Invention
The present invention relates to a spherical aberration correction apparatus for correcting aberration of a light beam.
2. Description of the Related Art
As a data recording medium wherein data recording or data reproduction is performed optically, an optical disc such as CD (Compact Disc) or DVD (Digital Video Disc or Digital Versatile Disc) is known. Various kinds of optical discs have been developed including a read-only optical disc, a write-once optical disc on which data can be recorded additionally, and a rewritable optical disc in which data erasing and rewriting can be performed. Furthermore, a multi-layer optical disclaimed to increase capacity of the optical disc is known, in which recording capacity for one side of the recording disc can be increased by providing multiple recording layers on a same recording side. Such multi-layer optical disc has a structure such that the multiple recording layers are stacked with a comparatively short, predetermined distance between two layers. For example, a recordable multi-layer optical disc using a recording medium such as a phase-change medium has been developed.
According to such improvement of recording density of the optical disc, research and development have been pursued on an optical pickup device and a data recording/reproduction device. Further, research and development have been made on an optical pickup device and a data recording/reproduction device that have compatibility for different types of optical discs.
In such an optical pickup device, a light beam such as laser light is irradiated on the optical disc during the recording or reproduction. It is important to correct or compensate aberration generated by reflection of a light beam reflected from the optical disc. The aberration changes during the recording or reproduction of the optical disc, since thickness of a cover layer of the optical disc generally has an in-plane distribution. In particular, the thickness of the cover layer from a disc surface to the recording layer (and thickness of a spacer layer) varies for each recording layer in the optical disc having the multiple recording layers on a same side of the disc. Therefore, the aberration changes when a reading position or recording position is changed from one recording layer to other recording layer. Further, a distribution of the aberration in a plane perpendicular to a light axis is changed as well as magnitude of the aberration.
As a conventional aberration correction apparatus for correcting such aberration, there is an apparatus using a beam expander which changes a beam diameter of the light beam, for example, an apparatus disclosed in Japanese Patent Application Kokai No.H10-106012. The aberration correction apparatus moves the beam expander along the light axis of the light beam, thereby correcting the aberration of the light beam caused by the optical disc. However, the apparatus has drawbacks that the beam expander causes an increased size of an optical system. In addition, a mechanism for driving a lens is necessary, which leads to a complicated structure.
As another method for reducing the effect of the aberration, conventionally, a pickup device having a liquid crystal element for correcting the aberration is proposed. For example, such a devise is disclosed in Japanese Patent Application Kokai No.H10-269911. The aberration correction element, having multiple phase adjusting portions formed concentrically, applies a predetermined voltage to each electrode, thereby adjusts an orientation state in the liquid crystal, and thus corrects the aberration generated by reflection of the light beam. However, in the aberration correction element having such configuration, thickness of the liquid crystal must be increased to correct a large aberration such as an aberration exceeding the wavelength of source light. In particular, in a case of use in the multi-layer disc, the thickness of the liquid crystal must be extremely increased, and a large applied voltage is required. Therefore, in addition to difficulty in fabrication, there has been a drawback of an extremely long response time required for a predetermined focal distance after application of voltage. This is because the response time is inversely proportional to the square root of the thickness of the liquid crystal layer. Therefore, there has been a problem that improvement of performance of the aberration correction device such as miniaturization, decrease of thickness, and speeding up is obstructed. Also, there is a problem that the increase of the thickness of the liquid crystal causes lowering of frequency response of the aberration correction device.